DESCRIPTION (Verbatim from the Investigator's Abstract): DiGeorge Syndrome is a complex of thymic dysplasia, cardiac defects, and parathyroid hypoplasia. In approximately 5 percent of patients, the thymic defect is so profound that the patients have no T cell function. In the clinical trial, complete DiGeorge patients are treated with thymic transplantation or unfractionated bone marrow transplantation. These unique patients will be used to test the following hypotheses: 1) Host thymopoiesis occurs in donor thymic allografts, 2) The function of transplanted thymus allografts (generation of new functioning T cells) can be assessed with minimally invasive methods [flow cytometry and T cell receptor (TCR)-rearrangement excision circle (TREC) analysis, 3) DiGeorge patients treated with HLA identical bone marrow, after redistribution of the adoptively transferred cells, will not show increases in these measures of recent thymic emigrants, and 4) in some patients, host CD3+ NK-T cells appear in the circulation prior to transplantation. Methods and Data Analysis: Given the small numbers of patients, these hypotheses will be addressed in a descriptive manner. Thymic allografts will be biopsied 3 months after transplantation and immunohistochemistry will be performed with monoclonal antibodies. The genetic origin of the various cell types in the graft will be determined by monoclonal antibody staining for relevant HLA antigens or in situ hybridization for sex chromosome markers. Functional analyses of peripheral T cells will include testing for cytokine production, flow cytometry, and proliferative responses to mitogens, monoclonal antibodies, antigens, and alloantigens (including the cells from the thymus donor). B cell function will be evaluated by antibody response to immunization and evaluation of somatic mutation in immunoglobulin genes. The minimally invasive methods used to determine whether new T cells are being formed in the transplanted thymus include the detection of TRECs by PCR and naive (CD45RA+CD62L+) T cells by flow cytometry. In patients treated with bone marrow transplantation, T cell function should result from adoptive transfer of donor T cells, not from thymopoiesis. Thus, after allowing for distribution of cells after bone marrow transplantation, we predict that the TRECs and naive phenotype cells should decrease with time. To determine if NK-T cells are present prior to transplantation, blood will be studied by flow cytometry looking for NK-T cell surface markers, by PCR to identify TCR rearrangements usually found in these cells, and in stimulation studies to assess cytokine production. These research studies done in parallel with the clinical trial of thymus and bone marrow transplantation in DiGeorge patients will greatly enhance our understanding of the role of the thymus in T, NK-T and B cell development.